This invention relates to an apparatus used for plasma processing of substrates.
In semiconductor fabrication processes, an enclosed process chamber having gas distributors for distributing process gas therein, is used to deposit material upon, etch, or implant material on substrates. A plasma is formed from the process gas using plasma generators comprising inductor coils, microwave plasma generators, or capacitive coupled process electrodes. Process gas byproducts are exhausted through an exhaust system. The gas distributor, plasma generator, and exhaust system often fail to provide a uniform distribution of reactive process gas or plasma species across the substrate surface. For example, process gas distributors positioned directly above the substrate can cause an asymmetric distribution of reactive process gas species across the substrate, with higher concentrations of reactive process gas at the center of the substrate and lower concentrations at the peripheral edge. Conversely, a process gas distributor positioned around the peripheral edge of the substrate can cause the concentration of reactive process gas or plasma to be higher at the peripheral edge than the center of the substrate. The distribution of process gas or plasma species can also be adversely affected by the nature of the magnetic fields formed by the plasma generator, or the asymmetric positioning of the exhaust nozzles that exhaust spent process gas byproducts from the chamber. It is desirable for the distribution of plasma species across the surface of the substrate to be uniform from the center to the peripheral edge of the substrate.
The use of process gas containment structures, such as a focusing ring around the substrate, are known to reduce such concentration gradients of reactive process gas, by containing the flow of process gas around the substrate. Typically the focus ring comprises a wall extending upwards in a plane transverse to the surface of the substrate, to encircle the substrate and contain the flow of process gas on the substrate. However, conventional plasma focusing rings often cause process gas to stagnate at the bottom wall of the focus ring next to the peripheral edge of the substrate, leading to the deposition of gas byproduct deposits on the focus ring. Such particulate contamination of the substrate can be reduced by periodically cleaning the process chamber components using a plasma of a corrosive gas, such as NF.sub.3. However, the corrosive plasmas erode the processing components and reduce the lifetime of the process chamber. Also, the cleaning process step increases down time of the apparatus and reduces process efficiency. Thus it is desirable to have a process gas distribution and containment system that reduces formation and deposition of contaminants on the substrate.
Additional non-uniformity problems arise in capacitively coupled chambers, where an electrically biased cathode electrode is located below a substrate, and the anode electrode is formed by electrically grounding the walls of the process chamber. The anode and cathode are maintained at differing electrical potentials to form an electric field that generates and/or accelerates plasma ions from the process gas toward the substrate. A strong and uniformly distributed electric field in the process chamber is needed to generate a uniform distribution of plasma ions for processing the substrate. However, conventional chambers often have a low surface area ratio of anode to cathode which results in a low density of plasma ions that does not allow efficient processing of the substrate. The substrate must remain in the process chamber for an extended period to provide sufficient exposure to the plasma ions, increasing process cycle time, and reducing process throughput. A low anode to cathode surface area ratio can also form a non-uniform distribution of electric field lines across the width of the chamber resulting in non-uniform processing of the substrate. The low surface area ratio is a particular problem when the anode or cathode surfaces are distant or removed from the substrate, for example, in process chambers in which the anode is formed by grounding the sidewalls, bottom wall, and/or ceiling of the process chamber.
Yet another problem arises when electrostatic chucks are used to electrostatically hold the substrate during processing. Electrostatic chucks comprise an electrostatic member electrically biased with respect to the substrate by an electrical voltage causing electrostatic charge to accumulate therein. In monopolar chucks, the plasma provides electrically charged species having opposing polarity to the substrate resulting in an attractive electrostatic force that holds the substrate to the chuck. Sufficient plasma ions are needed to accumulate electrically charged species in the substrate. A low anode to cathode surface area generates a low density level of plasma ions that result in insufficient charge accumulation and resultant weak electrostatic attractive forces that can cause the substrate to become misaligned during processing, resulting in loss of the entire substrate.
In electrostatic chucking systems, additional problems arise during dechucking or release of substrate from the electrostatic chuck. To dechuck a substrate, the residual electrostatic charge remaining after the supply of voltage to the electrostatic chuck is discontinued needs to dissipated to ground. In one method, the charge accumulated in the substrate is removed by bringing a grounded conductor in contact with the substrate. However, such discharging methods do not remove the charge carried by the plasma ions above the substrate. Charge from the charged plasma ions continues to accumulate on the substrate causing the substrate to remain electrostatically attracted to the chuck even after power to the chuck is turned off. During removal from the chuck, the substrate can become damaged due to opposing forces applied to the substrate by residual electrostatic attraction and by the substrate removal apparatus, such as lift pins. Alternatively, the substrate dechucking time must be increased to allow dissipation of the charge carried by the plasma ions by slow diffusional transport and leakage to ground through the process chamber walls, before attempts are made to remove the substrate.
Thus, there is a need for a plasma processing apparatus capable of maintaining a substantially uniform concentration of reactive gas across the substrate surface, and for reducing deposition of contaminants and process gas byproducts on the substrate. There is a further need for a plasma processing apparatus capable of providing a high anode to cathode surface area ratio for generating a high density of plasma ions. There is also a need for an apparatus that provides sufficient accumulation of charge on the substrate to securely hold the substrate on the electrostatic chuck, and is capable of rapidly discharging the plasma ions to allow fast dechucking of the substrate.